1. Field of the Invention
This invention relates to a driver circuit, a display device including a driver circuit formed over a common substrate with a pixel portion, or an electronic device including the display device.
2. Description of the Related Art
With the widespread use of large display devices such as liquid crystal televisions, display devices are required to be higher-value-added and thus under development. Particularly, a technique to configure a driver circuit such as a gate driver over a common substrate with a pixel portion using thin film transistors (TFTs) whose channel region is made from an amorphous semiconductor is actively developed because the technique helps a reduction in cost and an improvement in reliability a lot.
A thin film transistor whose channel region is formed from an amorphous semiconductor causes deterioration such as an increase in threshold voltage or a decrease in field-effect mobility. If deterioration in the thin film transistor proceeds, there arises a problem such that a driver circuit may have difficulty in operation and may fail to display an image. Therefore, a shift register capable of suppressing deterioration in a transistor is disclosed in Patent Document 1. In the Patent Document 1, in order to suppress degradation of transistor characteristics, two transistors are provided and the transistors are connected between an output terminal of a flip-flop and a wiring to which a low-voltage power supply potential VSS is supplied. Thus, the transistors become conducting in turn. In such a manner, a period during which each of the transistors is conducting can be shortened by approximately half in a frame period; therefore, degradation of transistor characteristics of the transistors can be suppressed to some extent.